1. Field of the Invention
The present invention relates to a porous sheet having a good hand and excellent durability.
More specifically, the present invention relates to a porous sheet obtained from a polyurethane having a specific chain structure and having excellent mechanical properties and durability. The porous sheet has excellent wet coagulatability upon its production and also excellent processability such as embossability upon embossing, and is suited for producing leather-like sheets.
2. Description of the Prior Art
A well known process for producing porous sheet is as follows: A solution of a polymer, principally comprising polyurethane resin, is applied on a support, such as a film or a fibrous structure such as nonwoven fabric, which is impregnated or coated with this solution. The obtained body is then immersed in a solution which is a non-solvent for the resin and miscible with the solvent of the resin solution, to coagulate, i.e. wet coagulation of the resin, whereby a porous sheet is obtained.
Porous sheets as obtained by the above process are, by being patched onto the surface of a fabric or, when it is integrated with a fibrous structure, used as it is, as leather-like sheets.
Porous sheets usable for leather-like sheets are required to maintain their strength without degradation upon long-periods of storage or use, i.e. required to be highly durable, flexible, in particular at low temperatures and to have a surface resembling that of natural leather, i.e. an excellent hand. To obtain this type of flexible, porous sheets having an excellent hand, it is necessary that a sufficiently porous state be achieved by wet coagulation (good wet coagulatability) and that, upon provision of the surface of greige porous sheet with a roughened surface structure by embossing, the desired surface morphology be given and, at the same time, the porous state not be damaged to a large extent (i.e., good embossability).
In general, a polyurethane is synthesized from a polymer diol such as a polyester diol, a polyether diol or a polycarbonate diol, a diisocyanate and a chain extender. With the obtained polyurethane, the component derived from the polymer diol is called the soft segment component and that from the diisocyanate and chain extender is called the hard segment component. The hard segments and soft segments are present alternately, to constitute the polyurethane. The soft segments give elasticity to the polymer, while the hard segments provide the strength and prevent plastic flow.
Japanese Patent Publications 10380/1984 and 10378/1984 describe a polyurethane, where the soft segment component is a polycarbonate diol, where the polyurethane has a good durability but poor wet coagulatability. The publications propose, to improve the above, by mixing two types of polyurethanes having nitrogen atoms contained in the isocyanate groups in different amounts by at least 0.2%, or by mixing at least two types of polyurethanes with polycarbonate diols having different molecular weights by at least 100. However, the resulting polyurethanes are only slightly, if ever, improved in the wet coagulatability and cannot give porous sheets having a good porous state.
Japanese Patent Application Laid-open No. 160820/1990 describes that a polyurethane resin composition, obtained by using as at least one diol component of a polycarbonate diol obtained from an aliphatic and/or alicyclic hydrocarbon diol has excellent mechanical properties and resistance to hydrolysis. However, the application gives no description of wet coagulating this polyurethane. In producing a natural leather-like sheet, the usual process comprises using, to provide the leather-like sheet with good flexibility, a nonwoven fabric comprising fibers from a plurality of polymers as a substrate layer and, after applying polyurethane to the substrate layer, extracting off with toluene, at least one of the polymers constituting the fibers. The polyurethane resin described in this application can, when wet coagulated and extracted with toluene, never give a porous sheet having a good porous state and smooth surface.
Japanese Patent Publication No. 29397/1973 describes that a polyurethane synthesized from a polyester diol, obtained from a mixed diol containing dipropylene glycol is good in both wet coagulatability and resistance to hydrolysis. However, the polyurethane from the dipropylene glycol-based polyester diol has poor durability. The publication further describes that: while polyurethanes using a polyester diol such as polyneopentyl adipate, polyhexene adipate, polybutylene sebacate, polyhexene neopentyladipate or polyhexene sebacate have good resistance to hydrolysis, they have poor wet coagulatability; and while polyurethanes using a polyester diol such as polyethylene adipate, polybutylene adipate or polydiethylene adipate have good wet coagulatability, they have poor resistance to hydrolysis, i.e. poor durability.
Japanese Patent Publication No. 14949/1991 discloses a fibrous material impregnated with a polycarbonate-based polyurethane and forming a microporous body by wet coagulation treatment. However, the microporous body obtained by this process can never be said to be of a good porous state. In fact, the process of the publication does not use the porous sheet obtained by wet coagulating the above polycarbonate-based polyurethane as a surface layer for which a good microporous state is required.
Japanese Patent Application Laid-open No. 244619/1991 discloses a process for producing a polyurethane resin which comprises reacting a polycarbonate polyol, an alicyclic polyisocyanate and an aromatic polyisocyanate. The application describes that, in particular, a combination of a polycarbonate polyol of polyhexamethylene carbonate diol, a alicyclic polyisocyanate of dicyclohexylmethane diisocyanate and an aromatic polyisocyanate of diphenylmethane diisocyanate gives a polyurethane having excellent mechanical properties, resistance to hydrolysis, light resistance and sweat resistance. However, the polyurethane obtained by the process has, same as the above, poor wet coagulatability and gives, on wet coagulation, a sheet having no microporous state, which is never flexible.
As stated heretofore, although use of a specific polyester diol such as polyethylene adipate diol as a soft segment component achieves good wet coagulatability, thereby giving a smooth surface, the obtained polyurethane has poor durability; and use of a polycarbonate diol or polyether diol as a soft segment component results in poor processability including wet coagulatability, thereby being inapplicable to the technical field of producing porous sheets. Furthermore, if conventional techniques can give good, flexible porous sheets at all, these sheets suffer deterioration in flexibility under low temperature conditions. Besides, these sheets are not quite satisfactory in processability such as embossability upon embossing treatment.
Accordingly, there remains a need for a porous sheet having good wet coagulatability and embossability, as well as excellent durability and flexibility, especially at low temperatures.